Flaxseeds are one of the richest dietary sources of the omega-3 fatty acid ALA. In fact, they are only exceeded by chia seeds ().

ALA is an essential fatty acid, which means that the body cannot produce it. Therefore, we need to obtain it from the food we eat.

Flaxseed oil contains the highest amount of ALA, followed by milled seeds. Consuming the seed whole provides the least amount of ALA, because the oil is locked up inside the fibrous structure of the seed ().

Because of their high content of omega-3 fatty acids, flaxseeds promote a lower ratio of omega-6 to omega-3. A lower ratio of omega-6 to omega-3 fatty acids may significantly reduce the risk of various chronic diseases (, ).

However, flaxseeds are not as potent as fish oils in providing omega-3. The ALA in flaxseeds needs to be converted into EPA and DHA in the body, a process that is often inefficient (, , ).

There is one variety that is not as nutritious as regular flaxseed, a yellow flax called "solin." It has a very different oil profile and is low in omega-3 fatty acids ().

Bottom line: Flaxseeds are rich in fat and are one of the best plant-based sources of heart-healthy omega-3 fatty acids.

p-Coumaric acid: This polyphenol is one of the main antioxidants found in flaxseeds.

Ferulic acid: This antioxidant may help to prevent several chronic diseases ().

Cyanogenic glycosides: These substances may form compounds called thiocyanates in the body, which can impair thyroid function in some people.

Phytosterols: Related to cholesterol, phytosterols are found in the cell membranes of plants. They have been shown to have cholesterol-lowering effects in the body ().

Lignans: Lignans are present in almost all plants, acting as both antioxidants and phytoestrogens. Flaxseeds are the richest known dietary source of lignans, containing up to 800 times more than other foods ().

Bottom Line: Flaxseeds are high in several plant compounds, including p-Coumaric acid, ferulic acid, cyanogenic glycosides, phytosterols and lignans.

Lignans

Flaxseeds are the richest known dietary source of lignans. These nutrients function as phytoestrogens ().

Phytoestrogens are plant compounds that are similar to the female sex hormone estrogen. They have weak estrogenic and antioxidant properties ().

They have been linked to a decreased risk of heart disease and metabolic syndrome, since they reduce the amount of fat and glucose in the blood.

Phytoestrogens also help lower blood pressure and reduce oxidative stress and inflammation in the arteries ().

Lignans are fermented by bacteria in the digestive system and may reduce the growth of several cancers, especially hormone-sensitive cancers such as breast, uterus and prostate cancers (, ).

Bottom line: Lignans are also known as phytoestrogens. They are antioxidants with weak estrogenic properties that are linked with benefits for heart health, metabolic syndrome and several types of hormone-sensitive cancers.

Flaxseeds have been associated with major benefits for heart health, mainly attributed to their content of omega-3 fatty acids, lignans and fiber.

Blood Cholesterol

High blood cholesterol is a well-known risk factor for heart disease. This is especially true for oxidized LDL-cholesterol ().

Human studies have shown that the daily consumption of flaxseeds, or flaxseed oil, may lower cholesterol levels by 6–11%.

These studies have also shown a 9–18% reduction in the number of LDL particles (, , , ).

This is supported by animal studies showing that flaxseeds may lower cholesterol levels and improve the composition of blood fats (, , , , ).

Flaxseeds may be very useful when consumed along with cholesterol-lowering medication. In one 12-month study, flaxseeds caused an additional 8.5% reduction in LDL-cholesterol, when compared to those not consuming flaxseeds ().

This cholesterol-lowering effect is thought to be caused by the high fiber and lignan content found in flaxseeds.

The fiber and lignans bind with cholesterol-rich bile acids and carry them down the digestive tract. This reduces cholesterol levels in the body ().

Omega-3 Fatty Acids

Omega-3 fatty acids are essential. They may have benefits for various aspects of heart health, including blood platelet function, inflammation and blood pressure.

Flaxseeds are very high in the omega-3 fatty acid alpha-linolenic acid (ALA).

They have been shown to decrease the risk of heart disease in animal studies by reducing inflammation in the arteries ().

Several studies have linked ALA with a lower risk of stroke, heart attacks and chronic kidney disease. These studies observed a 73% lower risk of sudden death as well, when compared to those with lower ALA intake (, , , ).

In one study, patients with heart disease were given 2.9 grams per day of ALA for one year. Patients receiving the supplement had significantly lower rates of death and heart attacks than those not taking ALA ().

Digestive Health

Diarrhea and constipation cause major distress and may even threaten health.

About 2–7% of people in the USA suffer from chronic diarrhea, while reoccurring constipation affects 12–19% of the population. This rate can be as high as 27% in Europe, with women at twice the risk of men (, ).

Several studies have found that flaxseeds actually prevent both diarrhea and constipation (, , ).

The content of insoluble fiber in flaxseeds adds bulk to the digestive waste, acting as a laxative and relieving constipation (, ).

It has also been proposed that the soluble fiber binds to water in the digestive tract. This causes it to swell and increase the bulk of the stool, therefore preventing diarrhea ().

Diabetes

According to the World Health Organization, 1 in 10 adults had diabetes in the year 2012 ().

In people with type 2 diabetes, studies show that supplementing 10–20 g/day of flaxseed powder for 1–2 months may reduce fasting blood sugar by up to 19.7% (, ).

However, not all studies have found flaxseeds to be effective in regulating blood glucose and insulin levels ().

Although the link between flaxseeds and type 2 diabetes is still unclear, they may be considered a safe and healthy addition to the diet for individuals with type 2 diabetes ().

Cancer

Test tube and animal studies have shown that flaxseeds may suppress the formation of several types of cancer, such as colon, breast, skin and lung cancer (, ).

Increased blood levels of sex hormones have been linked with an increased risk of several cancers (, , ). Flaxseeds may modestly lower serum levels of sex hormones in overweight women, and decrease the risk of breast cancer (, ).

Consumption of flaxseeds has also been shown to have protective effects against prostate cancer (, ).

Bottom line: Flaxseeds may improve digestion by relieving diarrhea and constipation. They may also reduce fasting blood sugar in diabetics and reduce the risk of several cancers.

Phytic Acid

Phytic acid is often referred to as an anti-nutrient since it may reduce the absorption of minerals like iron and zinc ().

However, phytic acid only impairs mineral absorption in a single meal, and does not affect any subsequent meals.

Therefore, this should not be a major concern except for individuals who are deficient in minerals like iron.

Digestive Problems

For people who are not used to eating a lot of fiber, incorporating flaxseeds too quickly can cause mild digestive problems. This includes bloating, gas, abdominal pain and nausea.

It is best to start with small doses and work your way up to one or two tablespoons daily.

Adding flaxseeds to the diet may also increase bowel movement frequency, since flaxseeds are a natural laxative.

Risks During Pregnancy

Although there are limited human studies on this, many health professionals fear that consuming flaxseeds during pregnancy may have undesirable effects.

This is due to the phytoestrogens that are present in flaxseeds, which may act similarly to the female sex hormone estrogen.

Animal studies have shown that flaxseeds and flaxseed lignans may cause lower birth weight and affect the development of the offspring's reproductive system, especially if consumed during early pregnancy (, ).

It is unlikely that smaller doses of flaxseeds will have an adverse effect. However, during pregnancy and lactation it is recommended to limit the intake of flaxseeds and other dietary sources of phytoestrogens. This also includes some soy products.

Blood Thinning Effects

Large doses of omega-3 fatty acids may have blood thinning effects ().

If you have a bleeding disorder or are taking blood thinners or other medications, consult with your doctor before incorporating large amounts of flaxseeds into the diet (, ).

Bottom line: Flaxseeds may cause mild digestive issues. They contain plant compounds that may adversely affect some people, and they can have blood thinning effects that affect some medications.